With the recent explosive growth in network traffic, ultra-high-speed optical transmission systems of 40 Gbit/s and more than 100 Gbit/s have been investigated. With respect to the ultra-high-speed optical transmission systems, active investigation has been performed on digital coherent communication obtained by combining a phase modulation method with coherent detection and digital signal processing technologies. The phase modulation method has better characteristics required for the long haul optical fiber transmission such as characteristics of the tolerance for signal light noise, chromatic dispersion, and polarization mode dispersion.
As a modulation method, Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK) have attracted attention because of their excellent tolerances for the dispersion compensation.
Moreover, in order to expand the transmission capacity without the increase in the frequency bandwidth, the research and development have been actively performed toward the practical use on a Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK) method and the like. The Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK) method is a method of multiplexing QPSK signals, which are superior in the frequency utilization efficiency, by two orthogonal polarizations.
An optical receiver for the digital coherent communication will be described below. It will be described here using the QPSK method as an example. With reference to FIG. 5, a process of receiving in the digital coherent communication will be described.
First, an optical receiver 30000 receives signal light in which a TE wave and a TM wave are multiplexed (hereafter, referred to as “TE-wave/TM-wave multiplexed signal light”). A local oscillation light source 32000 outputs local oscillation light in which a TE wave and a TM wave are multiplexed (hereafter, referred to as “TE-wave/TM-wave multiplexed local oscillation light”). The optical reception unit 31000 receives the TE-wave/TM-wave multiplexed signal light and the TE-wave/TM-wave multiplexed local oscillation light, splits each of them depending on the polarization, and makes the separated signal light and local oscillation light interference. The optical receiver 31000 outputs four signal light components in total which are composed of the real components and the imaginary components of each of the two signal light components, each of which has the polarization state parallel to each of two orthogonal polarization axes. The four signal light components are converted into analog electrical signals by an optical detector 33000, and then converted into digital electrical signals by an analog-to-digital converter 34000. These digital electrical signals are transformed by a re-sampling unit (not shown in the figure) into digital electrical signals which are sampled at the symbol rate (also referred to as a baud rate) of the signal light, and then inputted into a digital signal processing unit 35000. The digital signal processing unit 35000 has functions of compensating the chromatic dispersion, the polarization dispersion, and the phase noise and frequency deviation. For example, in compensating the optical carrier frequency deviation and optical phase deviation, the compensation is performed on a frequency deviation between the frequency of the received signal light and the frequency of the local oscillation light, and on an optical phase rotation due to an optical phase deviation, respectively. After that, each of the electrical signals is demodulated by a symbol decision unit 36000 into a bit sequence which an optical transmitter has transmitted.
In this way, the digital coherent detection in the ultra-high-speed optical communication system can be realized.
Hereinafter, the above-mentioned optical reception unit 31000 will be described in more detail. With regard to the optical reception unit, a study on the standardization has been conducted by the OIF (Optical Internetworking Forum), which is an industry organization to promote high-speed data communications, and optical reception units following the standard have been developed. There are various kinds of means for realizing the optical reception unit.
For example, Non Patent Literature 1 describes an example of a polarization demultiplexing unit in an optical reception unit realized by using a micro-optics technology. However, if the micro-optics technology is used in that way, it is difficult to adjust positional relationships between a plurality of bulk elements. Specifically, it is necessary to align the optical axes of the plurality of bulk elements, for example.
It is considered, therefore, that a silica-based planar optical integrated circuit (hereafter, referred to as a “planar lightwave circuit”) has promise as a means not requiring adjustments for positional relationships. Patent Literature 1 discloses an example of an optical reception unit realized by using the planar lightwave circuit. Patent Literature 1 discloses a configuration in which a groove is formed at a part of the planar lightwave circuit, and a photonic crystal chip is inserted to intersect a waveguide in order to make the photonic crystal chip function.